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A corner reflector
antenna for UHF TV
You can save quite a lot of money by building
your own UHF TV antenna and you will
generally get better performance into the
bargain. This corner reflector antenna covers
both UHF TV bands IV and V and should only
cost around $30 to $40 to build.
By BOB FLYNN & LEO SIMPSON
This is not the first UHF antenna
we have described. Back in the January 1988 issue of SILICON CHIP. we
described a 4-Bay Bowtie Antenna
which covered both UHF TV bands. It
has proved extremely popular and we
believe that many hundreds have been
made.
So why are we producing a different design and what was wrong with
the bowtie approach? Essentially,
there is nothing wrong with the bowtie design at all - it works very well
although it does require reasonable
metalworking skills. No, the real reason for presenting this new design is
simply that we wanted to assess a
large corner reflector design. We also
felt that it should be easier to build
than the bowtie design.
The corner reflector antenna is not
a very common design and is rarely
seen in Australia. Essentially, it consists of a single dipole element with a
large reflector immediately behind it.
The "corner" typically has an angle of
60-90 degrees.
Corner reflectors have most of the
advantages of bowtie arrays when
compared to the most common UHF
antenna - the long Yagi. The corner
reflector can be designed to cover a
wide frequency range, in this case
UHF bands IV and V, from 526582MHz and from 603-820MHz. It also
has a narrow vertical acceptance angle which is important for reducing
interference effects from aircraft (ie,
aircraft flutter and ghosting), and it
has a good front-to -back ratio.
Yet another advantage is that it can
be designed for high gain over the full
frequency range. The gain of the design presented here is close to 12dB
with respect to a simple dipole.
The disadvantage of the corner reflector design presented here is that it
is relatively large and cumbersome
and it does have relatively high wind
resistance. It will need a good strong
mast to ensure that it does not blow
down in high winds. This requirement applies to any large antenna, of
course.
Solid bowtie
This close up view of the antenna shows the triangular dipole elements attached
to the balun box. Note the pop rivets attaching the reflector elements to the two
booms. Alternatively, you can use stainless steel self-tapping screws.
24
SILICON CHIP
In our design, the dipole element is
like a large solid bowtie and is made
from two triangular sheets of aluminium. The reflector is essentially
two large grilles about 65cm wide and
60cm long which are mounted at
rightangles to each other.
Norie of the parts are difficult to
make although you will need some
basic metalworking tools. A guillotine would be nice although we did
not make use of one when we made
the prototype. At the very least, you
will need a hacksaw, an electric drill
and drill bits, a vise, tape measure or
long steel rule, various files, combi-
The corner reflector antenna works well on both UHF hand IV and hand V
frequencies and gives good reception from line-of-sight transmitters more than
100km distant. At this site in Carlingford (Sydney), good reception of
Wollongong stations was obtained.
nation square, a centre punch, scriber
and a certain amount of patience.
A pop rivet gun is also a necessity if
you are going to assemble the reflector sections with pop rivets.
Fasteners & hardware
Based on our own experience, you
are wasting your time making an antenna if you don't use the right hardware. The same applies when installing a ready-built antenna too, although
it is amazing how often you will see
antenna fittings severely corroded
after only a year or two of service.
Let's talk about fasteners first. Because aluminium is such an active
metal, 't he right fasteners must be used
otherwise corrosion will be very rapid,
especially in seaside areas. Use the
wrong types of screws in a seaside
area and they will rust out and cause
corrosion of the antenna itself in a
matter of only a few weeks or even
days. (We are especially aware of this
corrosion problem since we are situated near Sydney's northern beaches).
Therefore, we recommend only four
types of fastener for this project: (1)
Aluminium pop rivets with alumin-
ium mandrels. They may not pull up
as tight as those with steel mandrels
but steel mandrels rust out; (2) Stainless steel pop rivets; (3) Though often
hard to get, aluminium screws are
recommended although they are seldom available in self-tapping types
and so all screw holes would have to
be tapped; and (4) Stainless steel selftapping screws - these are strong, readily available and resist corrosion very
well.
Now for the junk we don't recommend (unless you are way out in the
country where it only rains once every
seven years). Avoid using galvanised,
bright zinc or cadmium-plated steel
screws. These might be OK for roofing but not for aluminium antennas.
Also , don 't use brass or mild steel
screws. When used in combination
with aluminium they corrode extremely rapidly. In fact, forget we even
mentioned brass or mild steel screws
- we know you're not silly enough to
use them!
The next item for consideration is
the antenna clamps. In the past, muffler clamps have often been used for
do-it-yourself antenna installations.
The sad fact is that many muffler
clamps are not even good enough for
mufflers let alone antennas. Try to
avoid those with bright zinc or cadmium plating. In seaside areas, the
nuts, washers and U-bolts (which are
highly stressed) are the first to corrode, followed by the brackets themselves. You have been warned - go for
the best antenna hardware you can
obtain. We prefer the use of galvanised U-bolts and V-clamps or, if you
want to be really fancy, go for ones
made of stainless steel.
The best place to buy such hardware is often not your local hardware
store or automotive accessory outlet
but a ships' chandler (ie, boating supplies). At least they know about corrosion problems and are likely to stock
suitable hardware, although their
prices can be expensive.
Making your antenna
Making and assembling this antenna is a fairly straightforward process although some steps do require a
little patience. You must first obtain
all the aluminium and hardware listed
in the Bill of Materials and make sure
you have access to all the tools listed
above.
Now let's just explain the various
JUN E 1991
25
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Fig.1: this diagram shows the corner reflector antenna in both front and side elevation. All the parts
labelled (A) to (F) are detailed in the other diagrams.
diagrams you need to refer to. Fig.1 is
a diagram showing the completely
assembled antenna in front and side
elevations. Note that each of the major parts is labelled and is shown in
detail on an additional diagram. For
example, note the reflector element
labelled (A). Twenty-two of these are
required and the required dimensions
are shown in Fig.2. Also shown in
Fig.2 is the reflector boom - shown as
(B) in Fig.1.
Mast mounting bracket
This edge view shows one of the mast mounting brackets which we fabricated
from 1.6mm sheet aluminium although they could also be made from angle
aluminium. Both pop rivets and screws were used in the prototype because it
had to be disassembled on quite a few occasions.
26
SILI CON CHIP
Part (C) in Fig.1 is the mast mounting bracket. This is shown in Fig.3
together with the dipole boom and
dipole boom bracket. Part (F) is half
the dipole which is detailed in Fig.4
together with the balun box drilling
diagrams. Fig.5 shows the detail of
the dipole boom, balun box and dipole when assembled together. Fig.6
is the circuit of the balun whil e Fig. 7
shows the wiring of the balun PC
board.
Having obtained all the materials,
you can start by cutting all the aluminium elements with a hacksaw.
First cut the two reflector booms (see
Fig.2) which are made of19mm square
tubing. One end of each boom is cut at
45 degrees, so you will need a combination square and a scriber to mark
the cut.
Centre punch and drill all the holes
on both booms. Make sure that all the
holes for the reflector elements are
positioned precisely as specified,
otherwise the reflector elements will
be crooked when assembled. The same
comment applies to the drilling of the
reflector elements themselves.
With the drilling of the reflector
metalwork complete, you can now pop
rivet both reflector grille assemblies.
When that is done, put them both
aside, well away from your work area
and where they are not likely to be
snagged as you pass by.
Incidentally, a point needs to be
made about the pop rivets. Nominally,
the aluminium pop rivets you buy
over the counter will all be 1/8-inch
or the metric equivalent 3.2mm, although some may be labelled as 3mm.
By contrast, stainless steel pop rivets
are available in 3mm and 3.2mm sizes.
Which ever size you buy, make sure
that you drill the correct size hole. Do
not drill 3.2mm holes for 3mm pop
rivets as they will just pull through.
"'"'
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Dipole elements
The dipole is made from two triangular sections of 1.6mm thick aluminium sheet - see Fig.4. Note: you
could use thinner material here, say
down to 1mm thick but you will still
need 1.6mm thick sheeting for the
mast brackets (or purchase angle section aluminium).
Cutting out the aluminium dipole
sections is one of the more tedious
steps in making this antenna and this
is where having access to a guillotine
would be good. Failing that, do it the
hard way using a hacksaw and then
file all the edges clean and straight.
Don't be tempted to use tin snips to
cut them out as it is very difficult to
avoid buckling the edges.
Two holes are required in each dipole half section, drilled at 3.2mm.
After doing the dipole sections, drilling the holes in the balun box will be
dead easy (see Fig.3 again) although
here's a little tip: don't drill at too
high a speed otherwise the plastic
"'"'
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45°
-1 ,o I.
I s s
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(A)
REFLECTOR
22 REQUIRED
MATERIAL:- 3mm x 10mm
ALUMINIUM
I
19
FRONT
SIDE
DIMENSIONS IN MILLIMETRES
(B) RELECTOR BOOM
2 REQUIRED
MATERIAL:· 19mm SQUARE x 1.15mm WALL
THICKNESS ALUMINIUM TUBING
HOLES:- A AND B: 3.2mm DIA. FOR POP RIVETS OR TO SUIT
AVAILABLE STAINLESS STEEL SELF TAPPING SCREWS
Fig.2: cutting and drilling details for the reflector booms and elements.
Note that one end of each reflector boom is cut at 45 degrees so that it
can be correctly clamped in the mast mounting brackets. Make sure
you drill the correct size hole for the pop rivets (see text).
will tend to melt around the holes.
booms. It requires holes to be drilled
at the top and sides, as shown in the
Next, cut and drill the dipole boom
which is made from the same 19mm detail diagram of Fig.3.
You have a choice as far as the
square tubing as used for the reflector ·
JUN E 1991
27
-·A
---· --- ,
29.5
SIDE
173
TOP
(0) DIPOLE BOOM
1 REQUIRED
MATERIAL:-19mm SQUARE x 1.15mm WALL
THICKNESS ALUMINIUM TUBING
15
25
12
16
33
40
73
40
(E)
DIPOLE BOOM TO
CONNECTION BOX BRACKET
·2 REQUIRED
MATERIAL:· 1.6mm ALUMINIUM
(C) ANTENNA TO MAST MOUNTING BRACKET
DIMENSIONS IN MILLIMETRES
HOLES:· A AND B: 3.2mm DIA. FOR POP RIVETS OR TO SUIT AVAILABLE
STAINLESS STEEL SELF TAPPING SCREWS
C: TO SUIT AVAILABLE MAST CLAMP U· BOLT
2 REQUIRED
MATERIAL:· 1.6mm ALUMINIUM
Fig.3: cutting and drilling details for the mast mounting brackets, dipole boom
and balun box brackets. Note that the mast mounting brackets could be made
from aluminium angle, which would save the need for bending sheet aluminium
and give a neater job.
mounting brackets are concerned. We
made ours from 1.6mm thick aluminium sheet as noted above, but you
can save yourself some work by buying a 300mm length of 32 x 32 x 3mm
aluminium angle. You will have to
modify the drilling dimensions
slightly for the mast U-bolts.
Also shown on Fig.3 is the small
rightangle bracket (2 required) needed
to connect the balun box to the dipole
boom. These can be made from a scrap
of aluminium.
Making the balun
The balun box provides a correct
Two antenna U-bolts and clamps are required for a
secure installation on the mast. The cable is passed
through the dipole boom and is anchored to the mast with
cable ties to stop it flapping in the wind.
28
SILICO N CHIP
termination for the antenna dipole
and terminals for the 75Q coax cable,
all sealed away from the elements for
protection. Inside is a PC board which
accommodates the air-cored balun and
75Q coax connection points.
The PC board measures 40 x 34mm
and is coded SC02106911 . It has a
very simple pattern. The balun is made
of two small coils of 0.7mm enamelled copper wire, as shown in Fig.6
and Fig.7.
Another view of the balun box, showing the four screws &
nuts attaching the dipole elements. Stainless steel screws
are mandatory here, since they attach to aluminium at
one end & the copper balun board at the other.
BILL OF MATERIALS
Antenna
1.4 metres of 19mm square
aluminium tubing with
1.15mm wall thickness
16 metres of 10mm x 3mm
aluminium flat (length based
on purchasing aluminium in 2
or 4-metre lengths)
1 350mm x 150mm x 1.6mm
aluminium sheet (see text)
60 pop rivets, 3.2mm x 10mm,
aluminium or stainless steel
mandrel; or
60 stainless steel self-tapping
screws (see text)
2 U-bolts & V-clamps to suit
mast
{F)
HALF DIPOLE
2 REQUIRED
MATERIAL:· 1.6mm ALUMINIUM
Balun Box
1 plastic zippy box, 80 x 52 x
30mm
1 PC board, code SC02106911
300mm x 0. 7mm diameter
enamelled copper wire
6 stainless steel machine
screws, 3mm x 12mm
12 3mm stainless steel nuts
<at>
<at>
A
A
15
BOX LID DRILLING FOR
DIPOLE MOUNTING
+A
15
15
<at>
15
<at>
(G)
Miscellaneous
75Q semi-air spaced coaxial cable
(Hills SSC-32 or equivalent),
plastic cable ties, silicone sealant,
Delrin plugs for square tubing .
t}- .
cp ~
.
DIMENSIONS IN MILLIMETRES
BOX BOTTOM DRILLING
~
HOLES:- A. 3.2mm DIA.
B: 8mm DIA.
Use wire with self-fluxing enamel
for this job. Self-fluxing enamel melts
easily in a solder pot or with a soldering iron and is much easier to work
with than high temperature wire
enamels which must be thoroughly
scraped off before the wire can be
tinned with solder. Wind the primary
coil on a 3.2mm former (such as a
drill bit) and then tin the ends. Similarly, wind the secondary coil on a
4.76mm former (a 3/16-inch drill) and
then tin the ends.
The primary coil fits inside the secondary coil before they are both soldered to the PC board. Incidentally,
do not think that the connections to
the outer coil, as shown on Figs.6 & 7
are a mistake. They are correctly
shown, with both ends soldered to
earth.
Fig.5 , the dipole and balun box assembly, shows how the PC board is
mounted inside the plastic box. Stain-
Fig.4: cutting and drilling details for the half dipole elements, and plastic
balun box. Note that thinner material could be used for the dipole halves,
down to 1mm thick. Cut them out using a hacksaw & then carefully
smooth the edges of each dipole section using a file.
(G)
BALUN
PCB
(!)
(D)
(F)
Fig.5: this diagram shows the assembly details for the dipole boom, balun
box and dipole elements. Note that stainless steel screws must be used for
the balun PC board connections. The balun box is attached to the boom
using two right-angle brackets.
]UNE 1991
29
BALUN
&
___1-----=r"'='=°="'..
_TO RECEIVER
ANTENN~-~PR_l
___
~
SEC
PRIMARY: 12T, 0.7mm ENAMELLED COPPER WIRE
CLOSE WOUND ON A 3.2mm DIA. "MANDREL
SECONDARY . 6T, 0. 7mm ENAMELLED COPPER WIRE CLOSEWOUND
ON A 4.76mm OIA. MANDREL
Fig.6: the circuit of the balun is
typical of commercial practice and
involves two concentric air-cored
windings with the secondary being a
shorted connection.
e
COILS MOUNTED ON COPPER SIDE OF BOARD
SECONDARY SLIPPED OVER ONE END OF PRIMARY
AND BOTH ENOS SOLDERED TO EARTH
Fig. 7: wiring details for the balun PC
board. Good quality semi-air-spaced
75Q coax with a woven copper shield
is mandatory here. Aluminium
shielded cable is not suitable for this
job since you can't solder to it.
less steel screws are used to attach the
dipole halves and the PC board to the
lid of the box. The box itself is attached to the antenna using the two
brackets depicted in Fig.3(E). Don't
fit the dipole/lid assembly to the box
at this stage - that can wait until the
antenna is about to be installed.
Final assembly
Now to put it all together. The two
reflector grilles and the dipole boom
are riveted or screwed together using
the two mast clamping brackets. The
idea is to pop rivet (or screw) the two
mast clamp brackets to one of the
reflector booms first. That done, pop
rivet in the dipole boom and finally,
the second reflector boom assembly,
so that the dipole boom is sandwiched
between the two reflector booms.
You can seal the ends of the reflector booms with square Delrin plugs
which are made for the job. At this
stage, you're just about finished.
Installation
Take a lot of care when installing
the antenna. There's no point doing a
wonderful job of making the antenna
if you end up in hospital after falling
off a ladder. Believe us, climbing ladders while carrying an antenna is dangerous work,
The first step in installation is to
decide where to mount your antenna.
For best results, mount it as high as
possible and well clear of trees and
other antennas.
It is not really practical to mount
this large corner reflector antenna on
the same mast as a VHF antenna un-
_J
Fig.8: the PC pattern for the balun box
is quite simple. You can either etch
your own board· or buy a ready-made
board from the usual suppliers.
less it is vertically separated by a distance of at least one metre. In any
case, if all goes well, you should be
able to dispense with your VHF antenna altogether.
Having erected your mast or J-pole
(for barge board mounting) you should
be ready to take the antenna up and
secure it with the U-bolts. But just
before you do that, you need to terminate the 750 coax cable. Pass the cable through the dipole boom and
through the balun box. Strip the cable
and tin the centre wire and shield,
then solder it to the relevant pads on
the PC board. Finally, place a cable tie
around the cable as a strain relief (as
shown in one of the photos) and then
screw the dipole/lid assembly onto
the case. You can now bolt the antenna to the mast.
Use good quality cable
This view of the balun box lid shows how the antenna cable is soldered to the
balun PC board. A plastic cable tie around the end of the cable stops it from
being pulled out of the box.
30
SILICON Cf/11'
By the way, for minimum signal
loss and good cable life, we recomm end Hills semi-air-spaced cable
(with holes in the cable dielectric),
type SSC-32 or equivalent. It's also a
good idea to keep this cable as short
as possible to minimise cable losses,
so try to site the antenna close to your
TV set.
Secure the cable to the mast with
plastic cable ties to stop .it flapping in
the wind. You may also like to seal
the balun box and the rear cable entry
to the antenna with silicone sealant,
to weatherproof it.
Depending on where you live, painting the antenna may be worthwhile,
particularly in seaside areas or industrial areas where there is a lot of fallout. In these cases, we suggest painting the antenna with an etch primer
and then finishing with an aluminium
loaded paint such as British Paints
"Silvar".
SC
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